There are plenty of tutorials on this effect. Try looking under the trapcode section of the tutorials here at creative cow. You can either have the entire person turn into particles at once, where you will just use your greenscreen footage as a layer source, or you will have a more complex effect where you precomp your footage and then use a mask or an alpha channel to sweep across the footage, turning a small portion of your footage into particles over time to disintegrate the whole image.

Hope this is not to complicated to follow: I am a bit frazzled right now.